1. Field of the Invention
This present invention relates generally to controlling the flow rate of a process gas into the reaction chamber of a processing system and, in particular, to an in-situ method for the estimation and verification of a liquid flow controller which is used to deliver liquid chemical precursors for semiconductor processing.
2. Description of Related Art
During a wafer fabrication process, the deposition formation rate on the wafer and the etching removal rate from the wafer depend on the input flow rate of the process gases and the pressure of the process gases in the chamber encapsulating the wafer. Changes in input flow rate of the gases create changes in chamber pressure.
A typical liquid delivery system for a chemical vapor deposition process includes: a liquid precursor controlled by a liquid mass flow controller to deliver the precursor to a vaporization device at a steady flow rate; a carrier gas, controlled by a flow controller, aim delivered to the vaporization device for mixing with the vaporized precursor; and one or more other process gases, controlled by flow controllers, delivered along with the carrier gas-vaporized precursor mixture to a reaction chamber. The liquid mass flow controller is controlled by a voltage signal, and oftentimes the factory calibration of the relation between the voltage signal and the flow rate is only approximate, with a tolerance up to 10% being acceptable. Accordingly, the in-situ flow rate must be adjusted to account for the imprecise calibration so as to produce wafers with the required uniformity.
An in-situ estimation and verification methodology to provide a compensated flow rate has been disclosed in U.S. Pat. No. 5,220,515. This procedure is based upon relating the actual liquid flow rate to chamber pressure by comparing the flow rate of the process gas with a known flow rate of a verification gas. The comparison relies upon the determination of the rate of change of both the verification gas pressure and process gas pressure, and ultimately a computation of the flow rate of the process gas into the chamber is expressed in terms of the flow rate of the verification gas and the rate of change of the pressures. Such a technique is complex and somewhat unwieldy because the computations rely upon effects occurring during transient behavior; computing derivatives of real data poses inherent difficulties, especially during transient periods, because of measurement inaccuracies and the data is typically corrupted by noise.
Moreover, the art is devoid of teachings or suggestions of how to compensate the flow rate of a liquid mass flow controller for a liquid precursor.